Apparatus and techniques for generating bunched ion beam

ABSTRACT

An ion implantation system, including an ion source, and a buncher to receive a continuous ion beam from the ion source, and output a bunched ion beam. The buncher may include a drift tube assembly, having an alternating sequence of grounded drift tubes and AC drift tubes. The drift tube assembly may include a first grounded drift tube, arranged to accept a continuous ion beam, at least two AC drift tubes downstream to the first grounded drift tube, a second grounded drift tube, downstream to the at least two AC drift tubes. The ion implantation system may include an AC voltage assembly, coupled to the at least two AC drift tubes, and comprising at least two AC voltage sources, separately coupled to the at least two AC drift tubes. The ion implantation system may include a linear accelerator, comprising a plurality of acceleration stages, disposed downstream of the buncher.

RELATED APPLICATIONS

This application is a continuation-in-part of and claims priority toU.S. patent application Ser. No. 16/842,464, filed Apr. 7, 2020,entitled NOVEL APPARATUS AND TECHNIQUES FOR GENERATING BUNCHED ION BEAM,which application is a continuation of and claims priority to U.S.patent application Ser. No. 16/107,151, filed Aug. 21, 2018, entitledNOVEL APPARATUS AND TECHNIQUES FOR GENERATING BUNCHED ION BEAM, whichapplications are incorporated by reference herein in their entirety.

FIELD OF THE DISCLOSURE

The disclosure relates generally to ion implantation apparatus and moreparticularly to high energy beamline ion implanters.

BACKGROUND OF THE DISCLOSURE

Ion implantation is a process of introducing dopants or impurities intoa substrate via bombardment. Ion implantation systems may comprise anion source and a series of beam-line components. The ion source maycomprise a chamber where ions are generated. The beam-line components,may include, for example, a mass analyzer, a collimator, and variouscomponents to accelerate or decelerate the ion beam. Much like a seriesof optical lenses for manipulating a light beam, the beam-linecomponents can filter, focus, and manipulate an ion beam havingparticular species, shape, energy, and/or other qualities. The ion beampasses through the beam-line components and may be directed toward asubstrate mounted on a platen or clamp.

One type of ion implanter suitable for generating ion beams of mediumenergy and high energy uses a linear accelerator, or LINAC, where aseries of electrodes arranged as tubes around the beam accelerate theion beam to increasingly higher energy along the succession of tubes.The various electrodes may be arranged in a series of stages where agiven electrode in a given stage receives an AC voltage signal toaccelerate the ion beam.

LINACs employ initial stages that bunch an ion beam as the beam isconducted through the beamline. An initial stage of a LINAC may bereferred to as a buncher, where a continuous ion beam is received by thebuncher and is output as a bunched ion beam in packets. Depending uponthe frequency of the AC voltage signal and the amplitude, the acceptanceor phase capture of an ion beam conducted through a known “double-gap”buncher using one powered electrode may be on the order of 30-35%,meaning that 65% of more of beam current is lost while being conductedinto the acceleration stages of the linear accelerator.

With respect to these and other considerations, the present disclosureis provided.

BRIEF SUMMARY

In one embodiment, an apparatus may include a multi-ring drift tubeassembly, including an alternating sequence of a set of grounded drifttubes and a set of AC drift tubes, arranged in alternating fashion withone another. The multi-ring drift tube assembly may further include afirst grounded drift tube, arranged to accept a continuous ion beam, atleast two AC drift tubes, arranged in series, downstream to the firstgrounded drift tube, and a second grounded drift tube, downstream to theat least two AC drift tubes. The apparatus may further include an ACvoltage assembly, electrically coupled to the at least two AC drifttubes. The AC voltage assembly may include a first AC voltage source,coupled to deliver a first AC voltage signal at a first frequency to afirst AC drift tube of at least two AC drift tubes; and a second ACvoltage source, coupled to deliver a second AC voltage signal at asecond frequency to a second AC drift tube of the at least two AC drifttubes. As such the second frequency may constitute an integral multipleof the first frequency.

In a further embodiment, an ion implantation system may include an ionsource to generate a continuous ion beam, and a buncher, disposeddownstream of the ion source, to receive the continuous ion beam andoutput a bunched ion beam. The buncher may include a drift tubeassembly, characterized by an alternating sequence of a set of groundeddrift tubes and a set of AC drift tubes, arranged in alternating fashionwith one another. The drift tube assembly may include a first groundeddrift tube, arranged to accept a continuous ion beam, at least two ACdrift tubes downstream to the first grounded drift tube; a secondgrounded drift tube, downstream to the at least two AC drift tubes, andan AC voltage assembly, electrically coupled to the at least two ACdrift tubes. The AC voltage assembly may include at least two AC voltagesources, separately coupled to the at least two AC drift tubes. The ionimplantation system may further include a linear accelerator, comprisinga plurality of acceleration stages, disposed downstream of the buncher.

In another embodiment, an apparatus may include a multi-ring drift tubeassembly and an AC voltage assembly. The multiring drift tube assemblymay include a first grounded drift tube, arranged to accept a continuousion beam and a first AC drift tube, disposed adjacent to the firstgrounded drift tube, and downstream of the first grounded drift tube.The multiring drift tube assembly may also include an intermediategrounded drift tube, arranged downstream of the first AC drift tube anddownstream of the first AC drift tube, and a second AC drift tube,disposed adjacent to the intermediate grounded drift tube, anddownstream of the intermediate grounded drift tube. The multiring drifttube assembly may also include a second grounded drift tube, wherein thesecond grounded drift tube is disposed adjacent to the second AC drifttube, and downstream of the second AC drift tube. The apparatus mayfurther include an AC voltage assembly, electrically coupled to themulti-ring drift tube assembly. The AC voltage assembly may include afirst AC voltage source, coupled to deliver a first AC voltage signal ata first frequency to the first AC drift tube, and a second AC voltagesource, coupled to deliver a second AC voltage signal at a secondfrequency to the second AC drift tube, wherein the second frequencycomprises an integral multiple of the first frequency.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows an exemplary ion implantation system, according toembodiments of the disclosure;

FIG. 1B shows another ion implantation system, according to embodimentsof the disclosure;

FIG. 2 shows an exemplary buncher, according to embodiments of thedisclosure;

FIG. 3 shows another exemplary buncher, according to other embodimentsof the disclosure;

FIG. 4 depicts results of modeling of operation of a drift tubeassembly, according to embodiments of the disclosure;

FIG. 5A and FIG. 5B are graphs illustrating the phase behavior ofdifferent rays of ion beams treated by different bunchers, highlightingbenefits of the present embodiments;

FIG. 6 depicts an exemplary process flow according to some embodimentsof the disclosure;

FIG. 7 shows another exemplary buncher, according to other embodimentsof the disclosure; and

FIG. 8 shows still another exemplary buncher, according to otherembodiments of the disclosure;

FIG. 9 shows yet another exemplary buncher, according to otherembodiments of the disclosure.

FIG. 10 shows a sawtooth waveform.

The drawings are not necessarily to scale. The drawings are merelyrepresentations, not intended to portray specific parameters of thedisclosure. The drawings are intended to depict exemplary embodiments ofthe disclosure, and therefore are not be considered as limiting inscope. In the drawings, like numbering represents like elements.

DETAILED DESCRIPTION

An apparatus, system and method in accordance with the presentdisclosure will now be described more fully hereinafter with referenceto the accompanying drawings, where embodiments of the system and methodare shown. The system and method may be embodied in many different formsand are not be construed as being limited to the embodiments set forthherein. Instead, these embodiments are provided so this disclosure willbe thorough and complete, and will fully convey the scope of the systemand method to those skilled in the art.

As used herein, an element or operation recited in the singular andproceeded with the word “a” or “an” are understood as potentiallyincluding plural elements or operations as well. Furthermore, referencesto “one embodiment” of the present disclosure are not intended to beinterpreted as precluding the existence of additional embodiments alsoincorporating the recited features.

Provided herein are approaches for improved high energy ion implantationsystems, based upon a beamline architecture. For brevity, an ionimplantation system may also be referred to herein as an “ionimplanter.” Various embodiments provide novel configurations forproviding the capability of generating high energy ions, where the finalion energy delivered to a substrate may be 300 keV, 500 keV, 1 MeV orgreater. In exemplary embodiments, a novel buncher design may beemployed to process an ion beam in a manner that increases theacceptance of the ion beam, as described below.

Referring now to FIG. 1A, an exemplary ion implanter, shown asimplantation system 100, is depicted in block form. The ion implantationsystem 100 may represent a beamline ion implanter, with some elementsomitted for clarity of explanation. The ion implantation system 100 mayinclude an ion source 102, and a gas box 107 held at high voltage asknown in the art. The ion source 102 may include extraction componentsand filters (not shown) to generate an ion beam 106 at a first energy.Examples of suitable ion energy for the first ion energy range from 5keV to 100 keV, while the embodiments are not limited in this context.To form a high energy ion beam, the ion implantation system 100 includesvarious additional components for accelerating the ion beam 106.

The ion implantation system 100 may include an analyzer 110, functioningto analyze a received ion beam. Thus, in some embodiments, the analyzer110 may receive the ion beam 106 with an energy imparted by extractionoptics located at the ion source 102, where the ion energy is in therange of 100 keV or below, and in particular, 80 keV or below. In otherembodiments, the analyzer 110 may receive the ion beam accelerated by aDC accelerator column to higher energies such as 200 keV, 250 keV, 300keV, 400 keV, or 500 keV. The embodiments are not limited in thiscontext. The ion implantation system 100 may also include a buncher 130,and a linear accelerator 114 (shown in the dashed line), disposeddownstream of the buncher 130. The operation of buncher 130 is detailedbelow. In brief, the buncher 130 is disposed downstream of the upstreambeamline 111, to accept the ion beam 106 as a continuous ion beam (or DCion beam), and to output the beam as a bunched ion beam. In a bunchedion beam the ion beam is output in discrete packets. At the same time,the energy of the ion beam may be increased by the buncher 130. Thelinear accelerator 114 may include a plurality of accelerator stages126, arranged in series, as shown. The accelerator stages 126 may actsimilarly to the buncher, to output bunched ion beams at a given stage,and to accelerate the ion beam to a higher energy in stages. Thus, abuncher may be considered to be a first accelerator stage, differingfrom downstream accelerator stages in that the ion beam is received as acontinuous ion beam.

In various embodiments, the ion implantation system 100 may includeadditional components, such as a filter magnet 116, a scanner 118 andcollimator 120, where the general functions of the filter magnet 116,scanner 118 and collimator 120 are well known and will not be describedherein in further detail. As such, a high energy ion beam, representedby the high energy ion beam 115, after acceleration by the linearaccelerator 114, may be delivered to an end station 122 for processingof a substrate 124.

In some embodiments, where the ion beam 106 is provided directly to theanalyzer 110, the buncher 130 may receive the ion beam 106 as acontinuous ion beam at a relatively lower energy, such as less than 100keV, as noted. In other embodiments, where the ion implantation systemincludes a DC accelerator column, the ion beam 106 may be accelerated tobe fed as a continuous ion beam at energies of up to 500 keV or greater.In these different cases the exact alternating current (AC) voltagesapplied by the buncher 130 may be adjusted according to the ion energyof the continuous ion beam received by the buncher 130.

FIG. 1B shows an embodiment of an ion implantation system 100 A,including a DC accelerator column 108, disposed downstream of the ionsource 102, and arranged to accelerate the ion beam 106 to produce anaccelerated ion beam 109 at a second ion energy, where the second ionenergy is greater than the first ion energy, generated by the ion source102. The DC accelerator column 108 may be arranged as in known DCaccelerator columns, such as those columns used in medium energy ionimplanters. The DC accelerator column may accelerate the ion beam 106,wherein the accelerated ion beam 109 is received by the analyzer 110 andbuncher 130 at an energy such as 200 keV, 250 keV, 300 keV, 400 keV, or500 keV. Otherwise, the ion implantation system 100A may functionsimilarly to ion implantation system 100.

FIG. 2 shows the structure of an exemplary buncher of a linearaccelerator, shown as buncher 130, according to embodiments of thedisclosure. The buncher 130 may include a drift tube assembly 150,including a first grounded drift tube 152, arranged to accept acontinuous ion beam, shown as accelerated ion beam 109. As shown, thefirst grounded drift tube 152 is connected to electrical ground. Thedrift tube assembly 150 may further include an AC drift tube assembly,arranged downstream to the first grounded drift tube 152. As discussedin detail below, the AC drift tube assembly 156 is arranged to receivean AC voltage signal, generally in the radio frequency range (RF range),which signal functions to accelerate and manipulate the accelerated ionbeam 109. In the embodiment of FIG. 2, the AC drift tube assembly 156includes just one AC drift tube. In other embodiments, an AC drift tubeassembly 156 may include multiple AC drift tubes.

The drift tube assembly 150 further includes a second grounded drifttube 154, downstream to the AC drift tube assembly 156. As a whole, thedrift tube assembly 150 is arranged as hollow cylinders to receive acontinuous ion beam, conduct the ion beam through the hollow cylinders,and accelerate some parts of the ion beam and decelerate other parts ina manner that bunches the ion beam into discrete packets, shown as bunch109A, to be received and further accelerated by an acceleration stage158, located downstream. The drift tube assembly 150 may be constructedof graphite or similar suitable material configured to minimizecontamination of an ion beam conducted therethrough. The subsequentaccelerating stages, indicated by acceleration stage 158, may operate ata well-defined frequency ω, and the capture of the bunches into thisaccelerating structure may be limited to approximately ±5° of phaseangle with respect to this fundamental angular frequency ω. In order totransmit the largest possible current through the entire beamline,arranging the buncher 130 to produce one bunch for each cycle of thisfundamental frequency ω is desirable.

As shown in FIG. 2, the buncher 130 further includes an AC voltageassembly 140, arranged to send to the AC drift tube assembly 156, an ACvoltage signal to drive a changing voltage at a powered drift tube ofthe AC drift tube assembly 156. The varying voltage on the AC drift tubeassembly 156 assembly provides different acceleration to the ions,depending on the arrival time of the ions at the AC drift tube assembly156. In this way, the trailing end 109A1 of the bunch 109A is given morevelocity than the leading end 109A2 of the bunch 109A, and the whole ofthe bunch 109A becomes as compact as possible when arriving at theacceleration stage 158. In various embodiments, the AC voltage signalmay be a composite of a plurality of individual AC voltage signals,superimposed to generate an AC voltage signal in a manner to provideimproved bunching of a continuous ion beam. In various embodiments, theAC voltage assembly 140 may generate a first AC voltage signal at afirst frequency, and a second AC voltage signal at a second frequency,where the second frequency comprises an integral multiple of the firstfrequency. In some embodiments, the AC voltage assembly 140 may generatea third AC voltage signal at a third frequency, where the thirdfrequency constitutes an integral multiple of the first frequency,different from the second frequency, and so forth. Thus, the secondfrequency, third frequency, etc., may be harmonics of the firstfrequency, where the frequency may be double, triple, etc., incomparison to the first frequency.

In the embodiment of FIG. 2, the AC voltage assembly 140 is shown togenerate three different AC voltage signals, represented by V₁cos(ωt+ϕ₁), V₂ cos(2ωt+ϕ2), and V₃ cos(3ωt+ϕ₃). For purposes ofillustration, the AC voltage signals are shown as sinusoidal signals,while other waveform shapes are possible. The AC voltage assembly 140may include a first AC voltage supply 142, second AC voltage supply 144,and third AC voltage supply 146, to generate a first AC voltage signal,second AC voltage signal, and third AC voltage signal, respectively. AnAC voltage supply may be embodied using an RF amplifier driven by asynchronized signal generator. The general term V refers to the maximumamplitude of the AC voltage signal while the general term ϕ refers tothe phase of the AC voltage signal. Thus, the maximum amplitude and thephase may differ among the different signals. In this embodiment, thesecond and third AC voltage signals represent a doubling and a tripling,respectively, of the frequency of the first signal w. As shown in FIG.2, the AC voltage assembly 140 may include an adder 148, where the adder148 sums the individual voltage signals and outputs a composite ACvoltage signal 149 to the AC drift tube assembly 156.

In various embodiments, the composite AC voltage signal may be formedfrom AC voltage signals where the highest frequency of an AC voltagesignal is approximately 120 MHz or less.

The composite AC voltage signal 149 is designed to adjust the phasedependence of ions processed by the AC drift tube assembly 156 in amanner that increases the acceptance at a downstream acceleration stage.In known linear accelerators of ion implantation systems, when acontinuous ion beam is bunched for transmitting in packets to downstreamacceleration stages, a certain fraction of the ion beam is lost to thewalls or other surfaces due to the nature of the acceleration andbunching process. The acceptance refers to the percentage of ion beam(such as a percentage of beam current) not lost, and therefore acceptedby the downstream acceleration stage. As noted, in known ionimplantation apparatus employing linear accelerators, the acceptance maybe on the order of 30% to 35% at a maximum, when various conditions areoptimized. Such known ion implantation systems may drive a buncher withan AC voltage signal having a frequency of 10 MHz, 13.56 MHz, or 20 MHz,with a voltage amplitude in the range of tens of kV. Notably, the ACvoltage signal in known ion implantation systems may be generated as asingle frequency, simple AC voltage signal.

Notably, the fundamental component of the composite AC voltage signalmay be simplified to V₁ cos(ωt), where the relative phase with respectto the other two AC voltage signals is given by the respective phaseoffsets, ϕ₂ or ϕ₃. As detailed below, these offsets may be adjusted toincrease acceptance.

In particular, the present inventor has found that the application ofmultiple frequencies to generate a complex (composite waveform)generates better output phase coherence/capture, as compared to knownbunchers that employ an AC voltage signal at a single frequency.

Turning to FIG. 3 there is shown the structure of an exemplary buncherof a linear accelerator, buncher 160, according to further embodimentsof the disclosure. The buncher 160 may include a drift tube assembly170, including a first grounded drift tube 182, arranged to accept acontinuous ion beam, shown as accelerated ion beam 109. As shown, thefirst grounded drift tube 182 is connected to electrical ground. Thedrift tube assembly 170 may further include an AC drift tube assembly180, arranged downstream to the first grounded drift tube 182. Asdiscussed in detail below, the AC drift tube assembly 180, similarly toAC drift tube assembly 156, is arranged to receive an AC voltagesignal(s), generally in the radio frequency range (RF range), whichsignal functions to accelerate and manipulate the accelerated ion beam109. In the embodiment of FIG. 3, the AC drift tube assembly 180includes three AC drift tubes, shown as AC drift tube 184, AC drift tube186, and AC drift tube 188.

The drift tube assembly 170 further includes a second grounded drifttube 190, downstream to the AC drift tube assembly 180. As a whole, thedrift tube assembly 170 is arranged as hollow cylinders to receive acontinuous ion beam, conduct the ion beam through the hollow cylinders,and accelerate the ion beam in a manner that bunches the ion beam intodiscrete packets, shown as bunch 109A, to be received and furtheraccelerated by an acceleration stage 192, located downstream. As suchthe drift tube assembly 170 may constitute a multi-ring drift tubeassembly having a length (along the direction of propagation of the ionbeam) of at least 100 mm and less than 400 mm.

In the embodiment of FIG. 3, an AC voltage assembly 162 is provided, andarranged to send to the AC drift tube assembly 180, an AC voltage signalto drive a changing voltage at a powered drift tube of the AC drift tubeassembly 180. The AC voltage assembly 162 may be configured whereinfirst AC voltage supply 142 drives the AC drift tube 184, the second ACvoltage supply 144 drives the AC drift tube 186, and the third ACvoltage supply 146 drives AC drift tube 188. These AC voltage signalsmay be synchronized in time by controller 164 to effectively generate acomposite signal similar to composite AC voltage signal 149. While FIG.3 illustrates a configuration where the lowest frequency AC voltagesignal is supplied to the furthest upstream AC drift tube, in otherembodiments, the lowest frequency AC voltage signal (V₁ cos(ωt+ϕ₁)) maybe applied to a different AC drift tube. The same applies to theintermediate frequency AC voltage signal (V₂ cos(2ωt+ϕ₂)), and the highfrequency AC voltage signal (V₃ cos(3ωt+ϕ₃)). This configuration has theadvantage over the configuration in FIG. 2 where the risk of a powersupply interfering with other power supplies is avoided.

While the use of a multiple-frequency AC voltage signal to drive abuncher is possible, notably, using multiple frequencies to generate anAC voltage signal may entail more voltage supplies, and may lead tolonger beamline, as detailed below. Accordingly, such a configuration ina beamline ion implanter has not heretofore been conceived. Notably, thepresent inventor has identified configurations where theseconsiderations may be overcome by adjusting drive signals to markedlyimprove ion beam throughput, especially for ions having a mass in therange of common dopants such as boron, phosphorous, and the like. Inparticular, in the “single-ring” (where “ring” refers to an AC drifttube) buncher of FIG. 2 or the “triple-ring” buncher of FIG. 3, acomposite AC voltage signal is produced, where the bunching of the ionbeam is performed in a manner to improve phase coherence by using of anion beam at a targeted distance from the AC drift tube assembly, andaccordingly to increase the acceptance.

Turning to FIG. 4, there is shown a composite illustration including adepiction of the drift tube assembly 150, and a corresponding phase map,shown as a function of distance in millimeters along the beam path. Thephase map is a graph illustrating the phase (shown on the right-handordinate) as a function of distance, with the position of the lone drifttube of the AC drift tube assembly 156, extending between 30 mm and 75mm. At this location, the voltage (shown by left hand ordinate) appliedto the AC drift tube assembly 156 reaches a maximum of approximately 18kV, and is applied at a frequency of 40 MHz. The relative phase positionof a series of 21 different rays of the accelerated ion beam 109 isshown to the right of the graph. The mass of the ions of the acceleratedion beam 109 is assumed to be 20 amu. As shown, the voltage reaches amaximum at the position of AC drift tube assembly 156, and is zeroelsewhere. At the point of entry into the AC drift tube assembly 156 the21 exemplary rays are equally spaced in phase at intervals of 18degrees. When treated by the composite AC voltage signal given by V=V₁cos(ωt+ϕ₁)+V₂ cos(2ωt+ϕ₂)+V₃ cos(3ωt+ϕ₃), such as generated by ACvoltage assembly 140, the various rays converge in phase to the right,as shown.

At a location corresponding to 700 mm, 670 mm to the right of theentrance to the AC drift tube assembly 156, the phase difference betweenmany of the rays is close to zero. Thus, when the entrance to anacceleration stage 158 is positioned at the 700 mm location,corresponding to a zero-phase difference between many of the rays, theacceptance may be a maximum. For an acceptance based upon +/−5-degreevariation, in the example of FIG. 4, the acceptance at the acceleratoris approximately 55%. In various other simulations, the maximumacceptance for the configuration of FIG. 4 has been calculated to be ashigh as 75%, a substantial improvement over the acceptance of 30%-35% inknown ion implanters employing single frequency bunchers. For example,when V is set to equal 59.4 kV, the acceptance is 75%, while at 24 kV,the acceptance is 65%.

Notably, the same behavior for phase convergence shown in FIG. 4 usingthe illustration of the AC drift tube assembly 156, may be obtained byapplying the same voltage parameters to the triple ring configuration ofthe AC drift tube assembly 180.

FIG. 5A and FIG. 5B are graphs illustrating the phase behavior ofdifferent rays of ion beams, highlighting the benefit of applying acomposite AC voltage signal in accordance with the present embodiments.FIG. 5A continues with the composite AC voltage parameters of theembodiment of FIG. 4, while FIG. 5B illustrates an example of applying asimple AC voltage signal to the ion beam. In the illustration of FIG.5B, the AC signal is given by V=V_(max) cos(ωt+ϕ), while in FIG. 5A theAC signal is given by V=V₁ cos(ωt+ϕ₁)+V₂ cos(2ωt+ϕ₂)+V₃ cos(3ωt+ϕ₃). Thefrequency ω is 40 MHz in both cases.

In the two different graphs, the phase behavior depicts the phase ofgiven rays at a designated distance from a point near the entrance tothe buncher as a function of the phase of the given rays at the entranceto the buncher. The designated distance is set at a distance where thephase of the different rays of the ion beam may be convenientlyconverged. Thus, with reference again to FIG. 4, in a bunch 109A,operation of the AC drift tube assembly 156 tends to accelerate thephase-lagging ions, trailing end 109A1, and tends to decelerate thephase-leading ions, leading end 109A2, leading to a phase convergence,such as at 700 mm.

In FIG. 5B, the most phase coherent condition, generating the highestrelative acceptance of 35%, there is a small degree of phase differenceat 400 mm even for differences in initial phase as small as 30 degrees.For other voltages the behavior is worse, as shown. Notably, theembodiment of FIG. 5A produces a convergence at 700 mm, somewhat longerthan the single frequency buncher results, requiring a convergence at400 mm. This result is in part due to the need to maintain AC voltageamplitude at a reasonable level for the composite AC voltage signal,such as approximately 20 kV. In the case of the single frequencybuncher, operation at 20 kV AC voltage amplitude allows convergence at400 mm. While the embodiment of FIG. 5A may entail a somewhat longerseparation between the buncher and accelerator in comparison to thesingle frequency buncher architecture (700 mm vs 400 mm), a benefit isthe substantially larger acceptance, and thus beam current, conductedinto the main accelerator stages of a LINAC. In various additionalembodiments, the convergence length may range from 300 mm to 1000 mm.

Without limitation as to a particular theory, the above results may beinterpreted in the following manner. The application of multiplefrequencies to generate a complex or composite AC voltage signal(waveform) may generate a waveform having a shape that is more conduciveto increasing capture. In principle a waveform having a sharpcharacteristic such as a vertical sawtooth shape, as shown in FIG. 10.This waveform may accelerate ions in a manner where one “tooth” causesthe ions to come together to make one bunch, in theory enabling ˜100%capture. Notably, in practical bunchers, a resonator based upon aresonance circuit is used to drive the AC voltage waveform at relevantfrequencies (in the MegaHertz range), where the resonance circuitinherently generates a sinusoidal waveform, which waveform does notgenerate high capture as in the vertical sawtooth case. In the presentapproach, the addition of sinusoidal waveforms at multiple differentfrequencies is used to generate a composite waveform that may exhibit ashape closer to the ideal sawtooth shape, and accordingly increase theimproved output phase coherence and capture, as described above.

Note that in the present embodiments, two or more waveforms may exhibitthe relationship where a first waveform is generated at a fundamentalfrequency and the other waveform(s) are generated at an integralmultiple of the fundamental frequency. In this manner, when a newcomponent is at an integral multiple of a fundamental frequency, eachion bunch will experiences the same fields, and the fundamental highestcommon factor frequency remains at the fundamental frequency.

While in principle the addition of a large number of waveforms (such asa Fourier series) may generate a synthesized composite waveform thatmore accurately approximates a sawtooth waveform, such an approach maybe impractical due to the increased cost of adding such a large numberof frequencies. The present inventor has discovered that the addition ofjust two or three harmonics of sinusoidal waveforms generates a verysubstantial increase in output phase coherence and capture, as discussedabove. Moreover, the present inventor has discovered that application ofdifferent sinusoidal waveforms to separate electrodes may work similarlyto applying the different sinusoidal waveforms to a single electrode,and that the application of just two waveforms generates a significantimprovement in output phase coherence and capture, similar to the caseof three waveforms, as opposed to the relatively lower output phasecoherence generated by a single frequency waveform.

While additional stages of a LINAC may perform in a similar manner tothe bunchers of the present embodiments, to accelerate and further buncha packet of ions, these additional stages of the LINAC need not bedriven by composite AC voltage signals as shown. In other words, becausethe composite AC voltage signal of the buncher has already mostlyconverged the phase of the various rays of the bunched ion beam at theentrance to an accelerator stage, further improvement of phaseconvergence may be less needed. This fact allows simpler design of ACvoltage assemblies to drive the accelerator stages of the LINAC.

As an example, in one embodiment of a triple-frequency composite ACsignal, the fundamental frequency for a first signal may be 40 MHz,while the first harmonic frequency may be 80 MHz for a second signal,added to the first signal, and a second harmonic frequency may be 120MHz for a third signal, added to the first signal and the second signal.

Notably, while the above embodiments emphasize generating composite ACvoltage signals based upon three AC voltage signals, and employing amulti-ring drift tube assembly including three drift tubes, in otherembodiments, a composite AC voltage signal may be formed from two ACvoltage signals, or four AC voltage signals. The embodiments are notlimited in this context. Likewise, a multi-ring drift tube assemblyaccording to other embodiments may employ two drift tubes or four drifttubes. The embodiments are not limited in this context.

FIG. 6 depicts an exemplary process flow 600 according to someembodiments of the disclosure. At block 602 an ion beam is generated asa continuous ion beam, such as by extraction from an ion source. Assuch, the ion beam may exhibit an ion energy in the range of several keVup to approximately 80 keV. Optionally, the continuous ion beam may beaccelerated to generate an accelerated continuous ion beam. In oneexample, a DC accelerator column may be applied to accelerate thecontinuous ion beam. As such, the accelerated continuous ion beam mayexhibit an ion energy of 200 keV to 500 keV or greater in someembodiments.

At block 604, the continuous ion beam is received in a multi-ring drifttube assembly. The multi-ring drift tube assembly may include a firstgrounded drift tube and a second grounded drift tube, as well as amulti-ring AC drift tube assembly, disposed between the first groundeddrift tube and the second grounded drift tube.

At block 606, the continuous ion beam is conducted through a first ACdrift tube of the multi-ring drift tube assembly while a first ACvoltage signal is applied at a first frequency to the first AC drifttube.

At block 608, the continuous ion beam is conducted through a second ACdrift tube of the multi-ring drift tube assembly while a second ACvoltage signal is applied at a second frequency to the second AC drifttube. In various embodiments, the second frequency may be an integralmultiple of the first frequency, such as double the first frequency. Inan optional operation, the accelerated continuous ion beam may beconducted through a third AC drift tube of the multi-ring drift tubeassembly while a third AC voltage signal is applied at a third frequencyto the third AC drift tube. The third frequency may be an integralmultiple of the first frequency, different from the second frequency. Assuch, the accelerated continuous ion beam may be output from themulti-ring drift tube assembly as a bunched ion beam.

FIG. 7 shows another exemplary buncher, for a linear accelerator,buncher 200, according to further embodiments of the disclosure. Thebuncher 200 may include a drift tube assembly 201, including a firstgrounded drift tube 202, arranged to accept a continuous ion beam, shownas accelerated ion beam 109. As shown, the first grounded drift tube 202is connected to electrical ground. The drift tube assembly 201 mayfurther include an AC drift tube assembly 203, arranged downstream tothe first grounded drift tube 182. The AC drift tube assembly 203,similarly to aforementioned AC drift tube assemblies, is arranged toreceive an AC voltage signal(s), generally in the radio frequency range(RF range), which signal functions to accelerate/decelerate andmanipulate the accelerated ion beam 109. In the embodiment of FIG. 7,the drift tube assembly 201 includes two AC drift tubes, shown as ACdrift tube 204, and AC drift tube 208.

The drift tube assembly 201 further includes a second grounded drifttube 210, downstream to the AC drift tube assembly 203. As a whole, thedrift tube assembly 201 is arranged as hollow cylinders to receive acontinuous ion beam, conduct the ion beam through the hollow cylinders,and accelerate/decelerate the ion beam in a manner that bunches the ionbeam into discrete packets, shown as packet 109B, to be received andfurther accelerated by a linear accelerator 212, located downstream. Assuch, the drift tube assembly 201 may constitute a multi-ring drift tubeassembly having a length (along the direction of propagation of the ionbeam) of at least 100 mm and less than 400 mm.

In the embodiment of FIG. 7, an AC voltage assembly 166 is provided, andarranged to send to the AC drift tube assembly 203, an AC voltage signalto drive a changing voltage at a powered drift tube of the AC drift tubeassembly 203. The AC voltage assembly 166 may be configured whereinfirst AC voltage supply 214 drives the AC drift tube 204, while thesecond AC voltage supply 216 drives the AC drift tube 208. In thisconfiguration and in the configuration of FIG. 8, the two different ACvoltage supplies may output a first frequency of 40 MHz and a secondfrequency of 80 MHz or alternatively, the two different AC voltagesupplies may output a first frequency of 13.56 MHz and a secondfrequency of 27.12 MHz according to different non-limiting embodiments.

These AC voltage signals may be synchronized in time by controller 164to generate similar beam behavior as produced by a single drift tubewith a composite signal given by V=V₁ cos(ωt+ϕ₁)+V₂ cos(2ωt+ϕ₂). In thismanner, the output phase coherence as a function of input phase of ionsmay be improved over single frequency bunchers in a manner analogous tothe embodiments of FIGS. 2-5B, discussed above.

While FIG. 7 illustrates a configuration where the lowest frequency ACvoltage signal is supplied to the furthest upstream AC drift tube 204,in other embodiments, the lowest frequency AC voltage signal (V₁cos(ωt+ϕ₁)) may be applied to a different AC drift tube.

FIG. 8 shows still another exemplary buncher, buncher 220, according toother embodiments of the disclosure. The buncher 220 may include a drifttube assembly 221, including a first grounded drift tube 202, arrangedto accept a continuous ion beam, shown as accelerated ion beam 109. Asshown, the first grounded drift tube 202 is connected to electricalground. The drift tube assembly 221 may further include an AC drift tube204, arranged downstream to the first grounded drift tube 202. In theembodiment of FIG. 8, an AC drift tube 208 is located downstream of theAC drift tube 204, and a second grounded drift tube 210 is locateddownstream to the AC drift tube 208, as in the embodiment of FIG. 7. Assuch, the drift tube assembly 201 may constitute a multi-ring drift tubeassembly having a length L (along the direction of propagation of theion beam) of at least 100 mm and less than 400 mm. In addition to theaforementioned components, the drift tube assembly 221 includes anintermediate grounded drift tube 206, disposed between the AC drift tube204 and AC drift tube 208. An advantage provided by this configurationis the reduced risk of cross-talk between the two power supplies (ACvoltage supply 214, AC voltage supply 216) and two resonant circuitsthat respectively drive the AC drift tube 204 and AC drift tube 208.

The embodiment of FIG. 8 illustrates a drift tube assembly 221characterized by an alternating sequence of one AC drift tubealternating with one grounded drift tube, as the ion beam in conducteddown a beamline. In other embodiments of alternating sequences, three ormore AC drift tubes may be provided to generate a composite AC signal,generally as described with respect to FIG. 3, except with a groundeddrift tube disposed between each successive pair of AC drift tubes. Inthis manner, cross-talk between all power supplies and resonators may becurtailed.

Note that in embodiments using two frequencies up to 200 degrees ofoutput phase coherence may be obtained with up to 55% acceptance of anion beam. In various embodiments, the tube length of drift tubes may beadjusted with the following considerations: 1) the length may beadjusted according to the distance the ions in a given ion beam travelin 180°, or

${D_{0} = \frac{\pi v}{\omega}},$

where v is the velocity. This distance gives the maximum accelerationfor a given voltage, but may produce some undesirable phase effects.Using shorter tubes as low as 0.2D₀ will require higher voltage but maygenerate better results overall. As regards the convergence length L,making this parameter shorter is beneficial but requires higher voltageto be applied. Accordingly, L may range from 300 mm to 1 m according todifferent embodiments based upon ion species, voltage considerations,and other effects.

Note also that while application of multifrequency signals may generallyact to increase convergence length when design is limited to a maximumvoltage applied and the individual frequencies are subtractive, specificmultifrequency designs may be accomplished without increasingconvergence length.

FIG. 9 provides an example of such an arrangement, where the buncher 230is shown. The drift tube assembly 232 includes a first grounded drifttube 234; a first AC drift tube 236, disposed adjacent to the firstgrounded drift tube 234, and downstream of the first grounded drift tube234; a first intermediate grounded drift tube 238, arranged downstreamof the first AC drift tube 236, a second AC drift tube 240, disposedadjacent to the first intermediate grounded drift tube 238, anddownstream of the first intermediate grounded drift tube 238; a secondintermediate grounded drift tube 242, disposed adjacent to the second ACdrift tube 240, and downstream of the second AC drift tube 240; a thirdAC drift tube 244, disposed adjacent to the second intermediate groundeddrift tube 242, and downstream of the second intermediate grounded drifttube 242; and a second grounded drift tube 246, wherein the secondgrounded drift tube 246 is disposed adjacent to the third AC drift tube244, and downstream of the third AC drift tube 244. Again, the provisionof the first intermediate grounded drift tube 238 and the secondintermediate grounded drift tube 242 may prevent crosstalk between thefirst AC voltage supply 142, second AC voltage supply 144, and third ACvoltage supply 146.

In sum, the present embodiments provide bunchers that are controlledusing multi-frequency signals applied either in concert to a lone ACdrift tube, or applied separately and individually to dedicated AC drifttubes. While not limiting various embodiments may employ commerciallyavailable frequencies as listed in table I below.

TABLE I Center Frequency Range frequency  6.765 MHz  6.795 MHz  6.78 MHz13.553 MHz 13.567 MHz  13.56 MHz 26.957 MHz 27.283 MHz  27.12 MHz  40.66MHz  40.7 MHz  40.68 MHz 433.05 MHz 434.79 MHz 433.92 MHz   902 MHz  928 MHz   915 MHz   2.4 GHz   2.5 GHz  2.45 GHz  5.725 GHz  5.875 GHz  5.8 GHz    24 GHz  24.25 GHz 24.125 GHz    61 GHz  61.5 GHz  61.25 GHz  122 GHz   123 GHz  122.5 GHz   244 GHz   246 GHz   245 GHz

The table I above illustrates various ISM frequencies, as defined by theUS FCC, where in the present embodiments, each frequency will be anintegral multiple of a fundamental frequency applied to one signal.Thus, in a two frequency embodiment, a combination of 13.56 MHz and27.12 MHz is suitable, in a three-frequency embodiment, a combination of13.56 MHz and 27.12 MHz, and 40.68 MHz is suitable, etc.

In view of the foregoing, at least the following advantages are achievedby the embodiments disclosed herein. A first advantage is realized byproviding a composite AC voltage signal to drive a buncher, so asubstantially larger ion beam current may be transmitted through a LINACdisposed downstream. A further advantage is the ability to drive a givenAC signal from a given power supply of a plurality of AC power suppliesto a dedicated electrode, avoiding interference between power suppliesthat may occur when coupled to a common multiple power supplies arecoupled to drive multiple AC voltage signals through a common electrode,while still driving larger ion beam current as in the case of acomposite AC voltage signal.

While certain embodiments of the disclosure have been described herein,the disclosure is not limited thereto, as the disclosure is as broad inscope as the art will allow and the specification may be read likewise.Therefore, the above description are not to be construed as limiting.Those skilled in the art will envision other modifications within thescope and spirit of the claims appended hereto.

1. An ion implantation system, comprising: an ion source to generate acontinuous ion beam; a buncher, disposed downstream of the ion source,to receive the continuous ion beam and output a bunched ion beam,wherein the buncher comprises a drift tube assembly, the drift tubeassembly comprising an alternating sequence of a set of grounded drifttubes and a set of AC drift tubes, arranged in alternating fashion withone another, the drift tube assembly further comprising: a firstgrounded drift tube, arranged to accept a continuous ion beam; at leasttwo AC drift tubes downstream to the first grounded drift tube; a secondgrounded drift tube, downstream to the at least two AC drift tubes; andan AC voltage assembly, electrically coupled to the at least two ACdrift tubes, the AC voltage assembly comprising at least two AC voltagesources, separately coupled to the at least two AC drift tubes; and alinear accelerator, comprising a plurality of acceleration stages,disposed downstream of the buncher to receive and accelerate the bunchedion beam.
 2. The ion implantation system of claim 1, wherein the ACvoltage assembly comprises: a first AC voltage source, coupled todeliver a first AC voltage signal at a first frequency to a first ACdrift tube of at least two AC drift tubes; and a second AC voltagesource, coupled to deliver a second AC voltage signal at a secondfrequency to a second AC drift tube of the at least two AC drift tubes,wherein the second frequency comprises an integral multiple of the firstfrequency.
 3. The ion implantation system of claim 2, wherein the firstfrequency is 40 MHz and the second frequency is 80 MHz.
 4. The ionimplantation system of claim 2, wherein the first frequency is 13.56 MHzand the second frequency is 27.12 MHz.
 5. The ion implantation system ofclaim 2, the buncher further comprising: the first grounded drift tube;a first AC drift tube, disposed adjacent to the first grounded drifttube, and downstream of the first grounded drift tube; an intermediategrounded drift tube, arranged downstream of the first AC drift tube; asecond AC drift tube, disposed adjacent to the intermediate groundeddrift tube, and downstream of the intermediate grounded drift tube; andthe second grounded drift tube, wherein the second grounded drift tubeis disposed adjacent to the second AC drift tube, and downstream of thesecond AC drift tube.
 6. The ion implantation system of claim 5, thebuncher further comprising: the first grounded drift tube; the first ACdrift tube, wherein the first AC drift tube is disposed adjacent to thefirst grounded drift tube, and downstream of the first grounded drifttube; a first intermediate grounded drift tube, arranged downstream ofthe first AC drift tube and downstream of the first AC drift tube; thesecond AC drift tube, wherein the second AC drift tube is disposedadjacent to the first intermediate grounded drift tube, and downstreamof the first intermediate grounded drift tube; a second intermediategrounded drift tube disposed adjacent to the second AC drift tube, anddownstream of the second AC drift tube; a third AC drift tube, disposedadjacent to the second intermediate grounded drift tube, and downstreamof the second intermediate grounded drift tube; and the second groundeddrift tube, wherein the second grounded drift tube is disposed adjacentto the third AC drift tube, and downstream of the third AC drift tube.7. The ion implantation system of claim 6, wherein the AC voltageassembly comprises a third AC voltage source, coupled to deliver a thirdAC voltage signal at a third frequency to the third AC drift tube,wherein the third frequency comprises an integral multiple of the firstfrequency, different from the second frequency.
 8. The ion implantationsystem of claim 7, wherein the second frequency is twice the firstfrequency, wherein the third frequency is three times the firstfrequency.
 9. The ion implantation system of claim 7, wherein the firstfrequency comprises a frequency of at least 13.56 MHz, and wherein thethird frequency comprises a frequency of 120 MHz or less.
 10. The ionimplantation system of claim 1, further comprising a DC acceleratorcolumn, disposed between the ion source and the buncher, and arranged toaccelerate the continuous ion beam to an energy of at least 200 keV. 11.An ion implantation system, comprising: an ion source to generate acontinuous ion beam; a buncher, disposed downstream of the ion source,to receive the continuous ion beam and output a bunched ion beam,wherein the buncher comprises: a first grounded drift tube, arranged toaccept a continuous ion beam; a first AC drift tube, disposed adjacentto the first grounded drift tube, and downstream of the first groundeddrift tube; an intermediate grounded drift tube, arranged downstream ofthe first AC drift tube and downstream of the first AC drift tube; asecond AC drift tube, disposed adjacent to the intermediate groundeddrift tube, and downstream of the intermediate grounded drift tube; asecond grounded drift tube, wherein the second grounded drift tube isdisposed adjacent to the second AC drift tube, and downstream of thesecond AC drift tube; and an AC voltage assembly, comprising: a first ACvoltage source, coupled to deliver a first AC voltage signal at a firstfrequency to the first AC drift tube; and a second AC voltage source,coupled to deliver a second AC voltage signal at a second frequency tothe second AC drift tube, wherein the second frequency comprises anintegral multiple of the first frequency and a linear accelerator,disposed downstream of the buncher, to receive and accelerate thebunched ion beam.
 12. An ion implantation system, comprising: an ionsource to generate a continuous ion beam; a buncher, disposed downstreamof the ion source, to receive the continuous ion beam and output abunched ion beam, the buncher comprising: a first AC drift tube toreceive a first AC signal at a first frequency; and a second AC drifttube, disposed downstream of the first AC drift tube, to receive asecond AC signal at a second frequency being an integral multiple of thefirst frequency; and a linear accelerator, disposed downstream of thebuncher, to receive and accelerate the bunched ion beam.
 13. The ionimplantation system of claim 12, further comprising a first groundeddrift tube, disposed upstream of the first AC drift tube; anintermediate grounded drift tube, arranged downstream of the first ACdrift tube; and a second grounded drift tube, wherein the secondgrounded drift tube is disposed adjacent to the second AC drift tube,and downstream of the second AC drift tube.